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Martin Weisskopf previewed agenda items, emphasizing the "what-flavor-chip" discussion. The ACIS Team has developed thin, back-illuminated CCDs with much better low-energy efficiency than front-illuminated devices, but with poorer energy resolution. The scientific issue then is, "How should one populate the ACIS-I and ACIS-S arrays?"
Alan Bunner discussed the state of the Agency. He foresees no threat to AXAF in the FY96 budget. Mr. Golden wants assurance that AXAF could operate beyond its 5-year mission goal. AXAF will carry sufficient expendables for at least 10 years of operation, and its orbital lifetime will be decades.
Fred Wojtalik gave (Enclosure 1) an overview of the Project. He recounted Mr. Goldin's phoned congratulations to the Project for its current record, especially the highly successful completion of the optics.
Gerry Flanagan summarized (Enclosure 2) several tasks which MSFC S&E is performing in support of AXAF. He described current studies to characterize the orbit and to determine communications availability.
Carey Riley reported (Enclosure 3) on the status of the XRCF. Major efforts underway include installation of components of the X-ray Source System (XSS), modifications to the liquid-nitrogen pumps, and fabrication of the thermal-control panels.
Max Rosenthal surveyed (Enclosure 4) the state of the Science Instruments. All are progressing well, except for the ACIS schedule. He outlined actions being taken to halt the slippage of schedule. The major concern is that ACIS will not be ready for calibration at the XRCF.
John Humphreys reported (Enclosure 5) on telescope development. Access is adequate for viewing the OTGs after integration and TRW is providing for late installation of x-ray shielding and magnetic brooms. The optics and the HRMA flight structure are complete. VETA-2 (assembly and alignment of P1-H1 into a flight-type mount) is serving as a pathfinder for the HRMA. Problems have been encountered and are being addressed; none seems insurmountable. Both MSFC and SAO now have full-time residents at Kodak. OCLI has all GSE and procedures in place for coating the optics.
Jean Oliver reported (Enclosure 6) on spacecraft development. Spacecraft charging presents some difficulties; none seems insurmountable. Jean summarized several changes the Project had approved for improving science performance: higher apogee, x-ray shielding, magnetic brooms, flux-scale transfer sources, the EPHIN radiation monitor, and safe-mode augmentation. Jean also noted that the current AXAF weight margin is small; however, he is confident that, after the next coupled-loads analysis and reduction of safety factors (from 1.5 to 1.25) at the CDA (Critical Design Audit), there will be no problem. If a weight problem persists, we may need a science trade to consider deletion of various x-ray shields, magnetic brooms, or the EPHIN. The Project has invested some of its reserve to accelerate the current schedule. The goal, made possible by early delivery of the optics, is to add two-months slack to the current schedule. Some members expressed concern that such an acceleration might increase risks, especially for the ACIS.
Louie Lofton reported (Enclosure 7) on operations development. AXAF will use DSN's 34-m dishes (at apogee), in addition to the 26-m dishes. The new AXAF orbit, with 140,000-km apogee and 10,000-km perigee (compared with the baseline's 100,000-km apogee), effects a 30% increase in nominal observing time, as measured by time above 60,000 km. In response to a question, he stated that end-to-end testing and operations will use the same command blocks.
Martin Weisskopf provided (Enclosure 8) some technical highlights since the previous SWG meeting. These included formulation of requirements for magnetic brooms and x-ray shielding and re-evaluation of the particle-radiation environments. The full simulation of focal-plane backgrounds is almost complete: The net external background (with addition of radiation shields) is comparable to the HRC internal background. There is a concern over the amount of proton shielding required for the Aspect Camera. Although there is a similar concern for ACIS, the ACIS proton shield is designed to a more realistic radiation environment.
Leon Van Speybroeck summarized (Enclosure 9) the state of the optics. The surface microroughness of the as-polished glass prior to coating ranges from 1.47 to 3.36 Angstroms, as measured with the MPMI. This significantly improves the high-energy performance of the telescope over specifications (and beyond goals!). SAO finds that the x-ray reflectance of the coating-qualification samples exceeds the specification.
Gordon Garmire provided (Enclosure 10) a preliminary magnetic-broom design with an explanation of why a magnetic broom may be necessary for AXAF.
Gordon Garmire summarized (Enclosure 11) the status of ACIS development. The first flight-like focal plane (but without flight-quality chips) has been assembled and sent to Martin Marietta. The back-illuminated-CCD development has been successful. Gordon concentrated on key issues for deciding the optimal focal-plane configuration of back-illuminated and front-illuminated devices.
Mark Bautz provided (Enclosure 12) details on ACIS device acceptance criteria and yields. He compared the performance of the back-illuminated devices with the front-illuminated ones. The principal scientific trade is enhanced efficiency at low energies, versus poorer energy resolution.
Paul Plucinsky described (Enclosure 13) simulations he had coordinated among ASC, ACIS, and HETG scientists. He noted that the front-illuminated devices perform better than originally expected. He requested feedback from the SWG to help reach a recommendation. A strong sentiment emerged for populating the imaging array and the central 2 elements of the spectroscopy array with different flavor CCDs.
Bert Brinkman summarized (Enclosure 14) LETG development. The prototype of the Grating-Element Support Structure (GESS) is so good that it will probably serve as a protoflight GESS. Bert requested clarification of the bake-out requirements for the LETG; he also suggested that the HRC-S might have a CsI coating on one end (with KBr over the remainder), to isolate better intrinsic spectral features from those in the coating response.
Steve Murray summarized (Enclosure 15) HRC development. The CDR was imminent and SAO had already received (from Galileo) 8 plates acceptable for flight. SAO has measured internal backgrounds as low as 0.04 counts cm^-2 s^-1.
Claude Canizares reported (Enclosure 16) the status of HETG development. Production of flight gratings was to begin in about 2 weeks. Claude also reported on test plans and preliminary results, including synchrotron measurements of grating efficiency.
Fred Seward described the ASC's preliminary recommendations on observing policy (a draft of which had already been distributed), in order to solicit the SWG's views. A strong SWG recommendation (supported by the Project Scientist) was that any plan to limit "GTO source monopolization" should treat the IDSs separately so that they can accomplish their observing programs. There was much discussion (but no consensus) about such plans --- e.g., limiting GTOs to the first 22 months, extending GTO time indefinitely (current NASA policy), and open competition for all proposals.
The meeting adjourned at 8:15 pm.
Horst Kunow described (Enclosure 17) the EPHIN (Electron, Proton, Helium INstrument), the SOHO flight spare which serves as AXAF's radiation monitor. Besides providing valuable diagnostic data, this instrument helps optimize AXAF observing time by signaling when focal-plane instruments can safely turn on after high-radiation events.
Martin Weisskopf announced the release of MSFC-2229 (requirements for the scientific calibration of AXAF), distributed copies to the members, and summarized (Enclosure 18) changes to the previous draft. The deadline for comments prior to baselining is April 3. (Changes after that date will be under formal configuration control.)
Martin Sulkanen described (Enclosure 19) a tool for modeling the spectra of electron impact sources. The tool provides factor-of-two accurate spectra, useful for filter selection and calibration planning.
Ron Elsner reported (Enclosure 20) on net system (source, filter, HRMA, and HRC) response, using results of the electron-impact-source model. Results for all XRCF source/filter combinations will be available on the WWW.
Steve O'Dell summarized (Enclosure 21) estimates for focal-plane backgrounds. Particle-induced backgrounds are likely to be comparable to the HRC's internal background, UV and cosmic-x-ray contributions will probably be relatively insignificant.
Brian Ramsey reported (Enclosure 22) on development of the forward radioactive sources, which provide a ground-to-orbit flux-scale transfer standard.
Roger Brissenden submitted the ASC status report (Enclosure 23). Impending events include meeting #3 (June) ot the ASC User Committee and submission (September) to NASA of observing-policy recommendations.
Ed Kellogg submitted the HXDS status report (Enclosure 24). The report includes (1994 October) CDR charts, which provides a good overview of the HXDS hardware.
Alan Bunner solicited ideas for using the AXAF polishing and metrology equipment at HDOS. Most of the SWG expressed concern that this equipment might not be kept available for AXAF, let alone preserving it for future missions.
The SWG then discussed preparation of a paper, for the Astrophysical Journal, describing AXAF and its potential use. Project Science will prepare an outline, identify potential simulations to be solicited/generated, and otherwise coordinate this effort.
The meeting adjourned at 11:30.
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Editor: Dr. Douglas Swartz System Administrator: Mr. Bob Dean Privacy Policy and Important Notices Accessibility Statement |
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